After you've installed phpBB, you need to perform a few post-install operations, namely to delete unneeded directories and to set some basic configuration options.
Connect to your server via FTP (skip this step for local installations).
Go to the directory where phpBB is installed.
Delete the
installandcontribdirectories.(Optionally) delete the
docsdirectory as well.
After a successful installation, when you visit your new phpBB forum's homepage you'll get a hint about your post-install tasks. You're advised to delete the install and contrib directories, because leaving them on the server is considered a security risk, so big that phpBB won't even run if it detects their presence.
The doc directory can be deleted as well if you like; it's not a security threat, but can save some disk space.